Thermocouples operating in accordance with the Peltier effect are well known in the arts. Applications for thermoelectric devices include heating, power generation and temperature sensing. However, the efficiency of previously known thermoelectric devices limited their usefulness.
As discovered by Peltier in 1835, arranging two dissimilar conductors next to each other and applying a voltage differential across the conductors can create a thermo electric device. More recently, thermoelectric devices have been formed with two dissimilar semiconductors, such as bismuth telluride (Bi2Te3) doped with selenium and antimony (Bi,Sb)2Te3 & Bi2(Te,Se)3 to form n-type and p-type materials. Other materials can include PbTe and SiGe. With a voltage applied across the two types of materials, the electrons in each material have a different potential energy. Therefore to move from one type of material to another type of material, the electrons must either absorb energy or release it, depending upon which direction they travel. The result is heat being absorbed on one side of the device and heat being released on the other, such as, for example, in the prior art device illustrated in FIG. 4.
FIG. 4 illustrates a cut away perspective drawing showing P-type 407 and N-type blocks 406 of material. As illustrated, like type blocks 406 or 407 are conductively connected via plates 403-405 and bordered by a packaging layer 408. Electrically conductive terminals 401 and 402 can extend beyond the package 408 for connection to a power source (not shown).
The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit of materials used in fabrication of the associated thermoelectric elements, ZT, where Z=α2/ρλ with α=the Seebeck coefficient (the change in voltage with temperature dV/dT), ρ=the electrical resistivity, and λ=the thermal conductivity. As can be seen from the definition of Z, the efficiency of a thermoelectric device decreases with increasing thermal conductivity or electrical resistivity. Improving the efficiency of thermoelectric devices requires either increasing the Seebeck coefficient or reducing the thermal conductivity or electrical resistivity.
It is known in the art to manufacture a thermoelectric device by extruding a billet of P-type material to form a P-type extrusion, also extruding a billet of N-type material to form an N-type extrusion. The P and N-type extrusions are sliced into wafers, the wafers are sliced into small elements, and the elements are mechanically loaded into a matrix of a desired pattern and assembled upon an electrically insulating plate with small copper pads connecting all of the elements electrically in series and thermally in parallel on the plate.
The prior art also includes methods of forming a thermoelectric material by combining a P-type extrusion with a N-type extrusion to form a P/N-type billet. The P/N-type billet may be extruded to form a P/N-type extrusion having P-type regions, and N-type regions. According to this method, the number of P-type regions and N-type regions correspond with the number of P-type extrusions and N-type extrusions used to form the P/N-type billet.
In some prior art embodiments, a thermoelectric module includes two ceramic substrate plates that serve as a foundation and also as electrical insulation for P-type and N-type Bismuth Telluride blocks. A pattern of blocks is laid out on the ceramic substrates so that they are electrically connected in series configuration. The position of the blocks between the two ceramic substrates provides a parallel configuration for the thermal characteristics of the blocks. The ceramic plates also serve as insulation between a) the blocks internal electrical elements and a heat sink that will typically be placed in contact with the hot side and b) the blocks internal electrical elements and whatever may be in contact with the cold side surface.
Typical commercially available modules have an even number of P-type and N-type blocks. The blocks are arranged so that one of each type of block shares an electrical interconnection often referred to as a “couple.”
As discussed above, it is known for P-type to be fashioned from an alloy of Bismuth and the N-type to be fashioned from an alloy of Tellurium. Both Bismuth and Tellurium have different free electron densities at the same temperature. P-type blocks are composed of material having a deficiency of electrons while N-type has an excess of electrons. As current flows through the module (up and down through the blocks) the amperage attempts to establish equilibrium throughout the module. The current causes the P-type material to become analogous to a hot area that will be cooled and the N-type to become analogous to a cool area that will be heated. Since both materials are actually at the same temperature, the result of the applied current is that the hot side of the module is heated and the cold side of the module is cooled. Since direct current is applied, the direction of the current can be used to determine whether a particular side of the module will be cooled or heated. Simple reversal of the DC polarity will switch the hot and cold sides.
However, the efficiency of the prior art hinder many applications. Much materials research has been conducted in an effort to find bulk materials with a higher figure of merit than Bismuth-Telluride, to no avail. As a result, recent efforts have focused on optimizing the thermoelectric device construction, rather than the basic materials. Unfortunately, each of the published methods have significant limitations, for example:                1) Superlattice Structures: The method reduces thermal conductivity in thermoelectric devices via blocking phonon conduction by constructing electron energy level barriers and requires an array of hundreds of precisely deposited thin layers of BiTe doped at slightly different levels. The transistion in doping levels between layers must be very sharp, making fabrication of such devices very expensive and also very sensitive to diffusion of dopants, leading to reliability problems.        2) Thermionic Emission devices: These devices cool via emission of electrons at a relatively higher energy level from one surface to another surface at very close proximity whose electrons are at a slightly lower energy level. This method requires the two surfaces be maintained approximately 10 nm apart using piezoelectric devices. It also requires the use of exotic materials such as cesium. They are very expensive to manufacture and unreliable due to the difficulty in maintaining a uniform 10 nm gap within the device.        3) Lateral thin film devices: These devices have thin film channels of BiTe deposited laterally on the surface of wafers (increasing the thermal conduction path length) and then rely upon heat transfer pads to conduct heat vertically through the device to the object being cooled in an effort to focus the conduction of heat in high-conductivity channels while limiting the parasitic losses elsewhere. These devices still suffer from thermal conductivity both laterally between channels and vertically through the device.        